Reaction metallurgical joining is a process in which a reaction material is heated and compressed between two metal workpiece substrates to facilitate the formation of a metallurgical joint between the substrates. The reaction material, in particular, is formulated to have a liquidus temperature below the lowest solidus temperature of the two metal workpiece substrates being joined and, additionally, to be reactive with the opposed faying surfaces of the workpiece substrates when disposed between those surfaces and heated above its solidus temperature. After being heated above at least its solidus temperature (and beforehand if desired), a compressive force is applied to the workpiece substrates, which squeezes and laterally spreads the reaction material, including any reaction by-products, along the faying interface of the workpiece substrates. The faying surfaces join together at this time to establish a low-resistivity solid-state metallurgical joint composed mainly of the base workpiece materials as the applied compression substantially expels the reaction material from the joint.
The reactivity of the reaction material enables coalescence without having to melt the metal workpiece substrates. Indeed, during reaction metallurgical joining, the reaction material forms a mobile liquid phase when heated above its solidus temperature, while melting of the facing workpiece substrates is typically avoided. This liquid phase breaks down surface films and materials—such as oxide films—present on the faying metal workpiece surfaces to expose clean portions of the faying surfaces, and can also locally dissolve a skin layer of each faying surface to make them temporarily more amenable to coalescence. The compressive force applied to the metal workpiece substrates—in addition to expelling the reaction material and any reaction by-products—eventually brings the cleaned portions of opposed faying surfaces into direct contact under pressure. A solid-state metallurgical joint ultimately results between the contacting coalescing portions of the opposed faying surfaces. Any leftover residual amounts of the reaction material still present at the faying interface simply re-solidify without substantially impacting the joint properties.
The heat input required to join the metal workpiece substrates by reaction metallurgical joining is relatively low compared to other joining techniques such as MIG welding, TIG welding, laser welding, and resistance spot welding, among others. Unlike those and other welding processes, which intend to generate enough heat to initiate melting of the base metals, reaction metallurgical joining can attain a solid-state joint directly between the metal workpiece substrates without having to generate such heat. As previously explained, reaction metallurgical joining inputs only enough heat to initiate melting of the reaction material, which in turn reacts with the workpiece substrate faying surfaces to initiate coalescence at a temperature below the temperature at which either of the workpiece substrates will begin to melt. The use of reaction metallurgical joining is thus an attractive option when heat-sensitive materials are located in close proximity to the metal workpiece substrates sought to be joined.
The reaction metallurgical joining process can be carried out in a number of ways. One common approach employs a pair of opposed electrodes to both resistively heat the reaction material above its solidus temperature and to compress the metal workpiece substrates. Specifically, the opposed electrodes, which communicate with a power source, impart a pressing force against oppositely-facing surfaces of the workpiece substrates in alignment with one another. The opposed electrodes are operated to pass an electrical current through the workpiece substrates and across their faying interface where the reaction material is present. The resistance to the flow of the electrical current through the reaction material and across the faying interface generates heat. This heat can be controlled to be within a zone that satisfies at least the following two conditions: (1) the heat is sufficient to initiate melting of the reaction material; and (2) the heat is not so excessive that the metal workpiece substrates themselves begin to melt. The opposed electrodes can also supply the compressive force needed to bring about the solid-state metallurgical joint between the contacting coalescing portions of the workpiece faying surfaces.